On 11 July 2012 11:39, Robbie Joosten <robbie_joos...@hotmail.com> wrote:
> @Ian: You’d be surprised how well Refmac can flatten sulfates if you have a > chiral volume outlier (see Figure 1d in Acta Cryst. D68: 484-496 (2012)). But this is only because the 'negative' volume sign was erroneously used in the chiral restraint instead of 'both' (or better still IMO no chiral restraint at all), right? If so I don't find it surprising at all that Refmac tried to flip the sulphate and ended up flattening it. Seems to be a good illustration of the GIGO (garbage in - garbage out) principle. Just because the garbage input in this case is in the official CCP4 distribution and not (as is of course more commonly the case) perpetrated by the user doesn't make it any less garbage. The point I was making is that in this and similar cases you don't need a chiral restraint at all: surely 4 bond lengths and 6 bond angles define the chiral volume pretty well already? Or are there cases where without a chiral restraint the refinement still tries to flip the chirality (I would fine that hard to believe). Seems to be a case of "belt and braces"! Cheers -- Ian
